Neutralizing circuit



March 21, 1944.

W VAN B. ROBERTS NEUTRALIZING CIRCUIT Filed Aug. 2, 1941 INPUT C/RC U/ T OUTPUT g OUTPUT am ur c/xecu/r V INVENTOR MLTER law's-(29527.9

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N EUTRALIZIN G CIRCUIT Walter van B. Roberts, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application August 2, 1941, Serial No. 405,163

11 Claims. (Cl. 179-471) tem of the aforesaid type wherein further means are provided for feeding direct current to the system at a. point of substantially zero radio frequency potential.

Still another object is to provide in such a system means for producing a desired amount of regeneration or degeneration.

Other objects of the invention will be apparent from a consideration of the following detailed description of the invention and the accompanying drawing.

Figure 1 of the drawing shows in diagrammatic form a. known form of neutralization employing an inductance to neutralize a capacity;

Figure 2 illustrates the equivalent circuit of Figure l at the operating frequency;

Figure 3 shows a network of a known type adapted to provide substantially zero transconductance at a single frequency between a pair of input terminals and a pair of output terminals;

Figure 4 shows, in accordance with the invention, the substitution of a network of the type shown in Figure 3 for the neutralizing arrangement shown in Figure 1 in a practical form .of amplifier;

Figure 5 shows a further modification of the invention wherein direct current feed is supplied at a point of low radio frequency potential; Fig. 5a shows a modified form of plate energizing circult for tube 20, and,

Figures 6 and 7 illustrate modifications of the arrangement shown in Figures 4 and 5 respectively and wherein sections of transmission lines are substituted for the inductance element.

Referring more particularly to Figure 1, there is shown therein an amplifier stage including an input circuit I, a triode amplifier 2 having inherent grid-anode capacityshown in dotted lines, an output circuit 3 and an inductance 4 connected in parallel with the grid-plate capacity.

The inductance 4 is chosen to have an admittance which, at the operating frequency, is equal in magnitude to the admittance of the grid-plate capacity of tube 2 so that the total current fed back from the output circuit 3 to the input circuit I is very small. At the operating frequency the combination in parallel of the inductance coil 4 and the grid plate capacity constitutes a very high resistive impedance as shown by element 5 in Figure 2. The input circuit, if composed of a parallel tuned circuit, also acts like a high resistance 5 connected between grid and cathode of tube 2 so that if voltage originates in the output circuit 3, it will causethe grid to take on a potential which is a real, positive, proper fraction of the output circuit voltage. In other words, there is degenerative feedback in the system and hence a greater driving power is required in the input circuit for a given output than would be the case if there were no feedback in the system.

The magnitude of this degenerative action may be illustrated as follows: Let us suppose that in normal operation the grid voltage amplitude is 400 volts and the output voltage amplitude is 1600 volts. Since these voltages are opposite in phase the total voltage amplitude between grid and plate is 2000 volts. Let Z represent the resonant impedance between grid and plate; that is, the magnitude of the resistance 5 shown in Figure 2. Then the amplitude of the current flowing through this resistance will be 2000/2.

More generally, let A represent the ratio of the voltages across output and input circuits. Then the current flowing through Z is times the grid voltage. Since this current flows from the grid, the effective impedance into which the grid voltage is working is equal to the grid voltage divided by this current or 1 A Thus, in addition to the input conductance of the tube itself, there is added the further conductance and it a d output terminals ii and i2, and comprising parallel resonant circuit 3, connected in series between one input terminal and one output terminal and a shunt resistance R connected between a point on this parallel circuit and the circuit connecting the other input and output terminals. It can be seen that ii t e shunt resistance R is infinitely high, an input voltage applied across input terminalsQ and Ill will produce a voltage of like phase on a load circuit connected across the output terminals ii and II, but if R is made zero, then the resonant circuit I, 8 will act as a voltage reversing transformer so that an input voltage applied across terminals 9 and ill will produce an opposite phase output voltage across output terminals I I, I12. It may therefore be expected that for any position of the tap l3 on the coil 5 shown in Figure 3, there wi11 be some value of resistance R which will result in substantially zero output voltage across output terminals ll, l2 as a result of voltage applied across terminals 9, Ill. If the inductance is considered as uniformly distributed along the coil 1, and if 1 represents the fraction or" the distance along the coil I from its left hand end at which R is connected by tap l3 then it can be shown that the value of R which is required to produce zero output voltage is equal to f(1-f)Z,' where Z is the anti-resonant impedance of the tuned circuit. It will be noted that by the reciprocity theorem the voltage may be applied at either pair of terminals without producing voltage at the other pair.

Assuming that resistance R has been chosen to have the value mentioned above, the input impedance measured between terminals 9, I is fZ while measured between terminals ll, l2 it .is (l--f)Z. These results will be referred to in what follows.

Referring now to Figure 4, there isshown a complete amplifier stage having shunt feed and having a network l6, I! of the type shown in Figure 3 connected between its output circuit l5 and its input circuit II. The blocking condensers I8 and is between the grid of the tube 20 and the grid input timed circuit l4 and between t e plate of the tube 20 and the output tuned circuit US, are assumed to have very low impedance at the operating frequency so that the neutralizing network will behave substantially as if connected directly between grid and plate. The neutralizing may be adjusted by varying the inductance connected between grid and plate circuits or by varying a small capacity i'l inlshunt thereto. To be specific, resistance R will be supposed to be connected to the middle of the inductance it so that j in the preceding expressions takes on the value one-half. In this case, the input admittance of the neutralizing network is in accordance with the previously derived expressions, equal to twice the admittance of the parallel tuned neutralizing circuit.

This admittance is. of course, in addition to the internal admittance between grid and cathode of the 'tube but it will be noted that this additional admittance is considerably less than the addltion-al admittance incurred in, the case of Figure 1. Furthermore, and also in accordance with the invention, the resistance R may be reduced or its point of connection to the coil shifted so as to introduce a small amount of regeneration, prefcrably not sufficient to cause oscillation under any conditions of circuit adjustment but at the same time sufiicieut to wipe out the increase-d loading of the input circuit mentioned above. Such adjustment is readily made experimentally.

While the arrangement shown in Figure 3 is simple and practical, it requires a choke of good characteristics for feeding direct current to the plate. In case the stage is intended for operation at any one of a number of widely different frequencies, a single choke may not prove satisiactory for all of such frequencies, This difiiculty is of course present in all shunt fed amplifiers. Therefore, in accordance with a further modification of the invention shown in Figure 5, the direct current feed is preferably made to a point on the neutralizing coil it at which the radio frequency potential is of a relatively low value. That such apoint must exist is evident from the fact that the radio frequency potentials of the two ends of this coil must be of opposite phase. In Figure 5, since the high D. C. voltage is connected to the neutralizing coil, a blocking condenser must be inserted between the neutralizing coil and its connection to the input circuit N. If the neutralizing coil I6 is made to have somewhat greater inductance than the value required for neutralizing, the blocking condenser 2i may be of sufficiently small capacity so that it may be a variable condenser and neutralizing may be adjusted by varying the capacity of this blocking condenser. Or if the blocking condenser is not variable, adjustment may be efiected either by means of a small variable capacity across the coil such as condenser II shown in. Figure 4, or

by varying the inductance slightly in any convenient manner.

It will be noted that a D. C. blocking condenser 22 is shown in series with R to reduce waste of power therein from direct current therethrough. However, condenser 22 may be rendered unnecessary by connecting the upper end of R to the positive rather than to the negative terminal of the source of plate voltage 23. This modification is shown in Fig. 5a.

In addition to the specific arrangements described above, other modifications are possible within the scope of the invention. For instance,

one such modification which is of particular interest in very short wave amplifiers comprises the use of a short length of transmission line to act as the neutralizing inductance. Circuits of this type are shown in Figures 6 and '7.

The circuit arrangement shown in Figure 6 is the equivalent of that of Figure 4, however, in Figure 6 a transmission line 24, 25 has been substituted for coil it of Figure 4:. The connection of one end of resistor R to the transmission line 24, 25 is made by means of slider bar it at such a point of the transmission line that the effective length of the transmission line is preferably less than one quarter wave length.

If it is desired to substitute a section of a transmission line for coil it of Figure 5, an arrangement such as is shown in Figure '2 may be used. In Figure 7, the lower part of the transrnission. line 2?. 28 is the equivalent of coil 8. The capacity between lines 27 and it replaces condenser 2i of Figure 5. This has been indicated in Figure 7 by the condenser 21' shown in dotted lines. The length of the line 21 2% is preferably between one quarter and one half wave length. A convenient way of facilitating the adjustment of the length cf line 21, 28 is to make the line elements in the form of telescoping tubes.

The radiation losses from lines 25 or 27, 28 may be reduced, if desired, by shielding preferably around the entire stage.

assayed While the various above disclosed embodiments of the invention are preferred, it is to be clearly understood that the invention is not limited to the details disclosed but may be varied within the scope of the following claims.

What I claim is:

i. In a radio frequency amplifying stage having an electronic tube provided with a plate, a

cathode and a grid, and input circuit and an output circuit for said tube, said tube having inherent grid-plate capacity, means providing neutralizing inductance connected effectively in parallel to said capacity thereby to neutralize said capacity, and resistive means, connected between a point of said first means and a point of substantially zero radio frequency potential, for reducing the residual degenerative feedback which results from inherent resistance in said inductance.

2. In a high frequency amplifying stage having I an electronic tube provided with a plate, a cathode and a grid, an input circuit and an output circuit for said tube, said tube having inherent grid-plate capacity, a neutralizing reactance connected effectively in parallel to said capacity thereby to neutralize said capacity, and a resistor connected to said reactance for reducing the residual degenerative feedback which results from inherent resistance in said inductance, said resistor being connected between the reactance and a point of relatively low alternating potential.

3. In a high frequency amplifier including a vacuum tube having inherent grid-plate capacity, an input circuit connected to the grid, an output circuit connected to the plate, an inductive reactance connected between the input circuit and the output circuit, said reactance being numerically equal to the reactance of said inherent capacity at the operating frequency. and a resistance connected between a point on said inductive reactance and a point of zero radio frequency potential, said resistance being substantially equal to the impedance of said inductive reactance and capacity reactance taken in parallel multiplied by the quantity f(1-f) where f is the ratio of the inductive reactance included, between one terminal of said inductive reactance and the point of contact between said resistance and said inductive reactance, to the total amount of said inductive reactance. g

4. In an amplifying stage including a vacuum tube having at least an anode, a cathode, and a grid, resonant input and output circuits for said tube, said output circuit being connected between said anode and cathode, an inductance coil independent of said output circuit having one end connec' d directly to the plate, a blocking condenser wnnecting the other end of said inductance to the input circuit, and a connection for feeding direct current for said plate into a point on said inductance coil at which the radio irequency potential is low as compared to the radio frequency potential at the plate.

5. In a high f equency amplifier including an electronic tube having an anode, a cathode, and a grid, an input circuit and an output circuit for said tube. an inductance coil connected between the anode and grid, a direct connection between the anode of said tube and a point of said inductance coil, a blocking condenser connected between another point of the coil and a point of said input circuit, a source of direct current having its positive terminal connected to a point on said inductance coil at which the radio frequenc potential is low as compared to the radio frequency potential at the anode and its negative terminal connected to the cathode of said tube, and a condenser and resistance in series connected between a point of said inductance coil and said cathode.

6. In a radio frequency amplifier including a vacuum tube having a plate, a cathode, and a grid, said tube having inherent grid-plate capacity, an input circuit connected to said grid, an output circuit connected to the plate, an inductive reactance having one end connected to the output circuit and its other end connected to the input circuit through a blocking condenser, said reactance being numerically equal to the reactance of said inherent capacity at the operating frequency, a connection including resistance between a point of said inductive reactance and apoint of zero radio frequency potential, said resistance being substantially equal to the impedance of said inductive reactance and capacity reactance taken in parallel, multiplied by the quantity f(1-,f) where I is the ratio of the inductive reactance included between one termi-' nal of the inductive reactance and the point of contact between said connection and said inductive reactance, to the toal amount of said inductive reactance, and a connection for feeding direct current for said plate into a point on said inductance coil at which the radio frequency potential is low as compared to the radio frequency potential at the plate.

7. In a radio frequency amplifying stage, an electronic tube having a plate, a cathode, and a grid, an input circuit and an output circuit for, said tube, said tube having inherent gridplate capacity, a neutralizing reactance connected effectively in parallel to said capacity to neutralize said capacity, said reactance comprising a section of a transmission line one conductor of which is connected to the input circuit and the other conductor of which is connected to the output circuit, said line being open circuited and having a physical length between A and of a wave length.

8. In a radio frequency amplifying stage, an electronic tube having a plate, a cathode, and a grid, an input circuit and an output circuit for said tube, said tube having inherent grid-plate capacity, a neutralizing inductance connected effectively in parallel to saidcapacity to neutralize said capacity, said inductance comprising a section of a transmission line one conductor of which is connected to the input circuit and the other conductor of which is connected to the output circuit, and means comprising a resistance having one end connected to said transmission line and its other end connected to a point which is substantially at zero radio fre quency potential, said resistance acting to reduce the residual degenerative feedback which results from inherer resistance in said transmission line. Y

9. The arrangement described in the next preceding claim 8 wherein the length of said transmission line is less than one quarter wave length of the operating frequency.

10. In a high frequency amplifier, a vacuum tube having a plate, a grid, and a cathode, said tube having inherent grid-plate capacity, an input circuit connected to the grid, an output circuit connected to the plate, an inductive reactance comprising a transmission line having one conductor connected to the input circuit and another conductor connected to the output circuit, said inductive reactance being numerically equal to the reactance of said inherent capacity at the operating frequency, a, Source of plate potential having a positive terminal and a negative terminal, means for connecting the positive terminal to a point of the conductor which is connected to the output circuit and which point is at a low radio frequency potential as compared to the radio frequency potential at the plate of said tube, means for connecting the negative terminal of said source of direct current to a point of zero radio frequency potential and a resistance means connected between another point of said last named conductor and a point of zero radio frequency potential.

11. In a radio frequency amplifying stage, an electronic tube having a plate, a cathode, and

a grid, an input circuit and an output circuit for said tube, said tube having inherent grid-plate capacity, a neutralizing reactance connected ei fectively in parallel to said capacity to neutralize said capacity, said reactance comprising a section of a transmission line one conductor of which is connected to the input circuit and the other conductor of which is connected to the output circuit, said line being between and V2 of wave length long, and means comprising a resistance having one end connected to said transmission line and its other end connected to a point which is substantially at zero radio frequency potential, said resistance acting to reduce the residual degenerative feedback which results from inherent resistance in said transmission line.

WALTER VAN B. ROBERTS. 

